In recent years, an NdFeB (Nd—Fe—B based) permanent magnet having excellent magnetic characteristics has been developed that enables high power and size reduction of a motor, and the scope of its use for various electronic appliances, electric cars, and vehicle motors is gradually increasing.
In general, magnetic characteristics of a magnet can be expressed as residual magnetic flux density and coercive force, and herein the residual magnetic flux density is determined by the fraction, density, and magnetic orientation degree of a NdFeB main phase. The coercive force is the durability of the magnetic force of a magnet caused by external magnetic field or heat, and it has a decisive relation with the microstructure of a tissue. The coercive force is determined by refining crystal grain size or homogeneous distribution on a crystal grain boundary.
In order to improve such coercive force, magnetic anisotropy energy is generally increased by adding a rare earth element such as Dy and Tb instead of Nd. But rare earth elements such as Dy and Tb are very expensive, and therefore, cause the total price of the permanent magnet to increase, and reduce the price competitiveness of the motor.
Thus, many other methods for improving the coercive force of a permanent magnet have been developed. For example, a binary alloy method for manufacturing a magnet by mixing different kinds of alloy powder having binary composition, forming a magnetic field and sintering thereof.
For example, a magnet may be manufactured by mixing Re—Fe—B powder (herein Re is rare earth) including a rare earth element such as Nd or Pr, and alloy powder. Residual magnetic flux density reduction may be inhibited when the added element of the alloy powder is distributed around the grain boundary of a Re—Fe—B crystal grain but very little of the element is on the grain boundary, thereby embodying high coercive force. However, this method has a problem in that the element of the alloy powder may diffuse into the particle when sintering. Thus, the effect may be reduced.
Recently, a method of sintering the Nd—Fe—B permanent magnet followed by diffusing a rare earth element from the magnet surface into the grain boundary has been developed, and this method is called a grain boundary diffusion method.
The grain boundary diffusion method is performed by forming a film by evaporating or sputtering a rare earth metal and the like on the Nd—Fe—B magnet surface followed by heating thereof, or by coating a rare earth inorganic compound powder on the sintered body surface followed by heating thereof. The rare earth atom deposited on the sintered body surface diffuses into the sintered body by heat treatment via a grain boundary part of the sintered body composition.
Accordingly, it is possible to concentrate the rare earth element at very high concentration on the grain boundary part or around the grain boundary part inside the sintered body main phase grain, and therefore, a more ideal tissue is formed than in the case of the binary alloy method described above. Furthermore, the magnetic characteristics reflect this tissue form, and maintenance of residual magnetic flux density and high coercive force are more notably expressed.
However, in the grain boundary diffusion method, there are many problems when using the evaporation or the sputtering method for mass production, and this may lead to decreased productivity.
In addition, the method of coating rare earth inorganic compound powder on the sintered body surface, and then heating thereof is a very simple coating process, compared to the sputtering or the evaporation method, and it has an advantage of high productivity, i.e., there is no deposition between magnets even when charging work pieces on a large scale during processing. However, there is a disadvantage in that the rare earth element diffuses by a substitution reaction between the powder and the magnet ingredients, so it is difficult to introduce them into the magnet in a large quantity.
On the other hand, a method of mixing calcium or calcium hydride powder to the rare earth inorganic compound powder and coating thereof on a magnet has also been developed, and in this method, the rare earth element is reduced by calcium reduction reaction during heat treatment and then diffused. This is an excellent method in terms of introducing the rare earth element on a large scale, but it has disadvantages in that handling of the calcium or calcium hydride powder is not easy and productivity may be lowered.
Regarding the grain boundary diffusion methods, one technique attaches the rare earth element to the NdFeB sintered magnet surface in order to prevent a reduction of coercive force, which is reduced when the NdFeB sintered magnet surface is processed for the purpose of thinning and the like, but there is a problem in that the coercive force improvement effect is insufficient.
Further, there is a technique of inhibiting irreversible demagnetization generated at high temperature by diffusing the rare earth element on the NdFeB sintered magnet surface, but this also demonstrates insufficient improvement in the coercive force.
In addition, the method of attaching the ingredients containing the rare earth element on the magnet surface by the sputtering method or the ion plating method has a disadvantage in that it is not practical due to high processing cost.
The method of coating the rare earth inorganic compound powder on the magnet base surface has an advantage of low processing cost, but it has a problem in that the degree of coercive force improvement is not very high, or the effect is not uniform. In particular, the rare earth inorganic compound prevents the diffusion of the pure rare earth element into the grain boundary diffusion, and then the rare earth inorganic compound remains inside the magnet, thereby the coercive force improvement is limited. And, processing for removing an oxidized film on the magnet surface after grain boundary diffusion has problems in that it causes a limitation on the grain boundary diffusion process such as reduction of diffusion depth, and increases the amount of processing when manufacturing a magnet.
The above information disclosed in this Background section is only for the enhancement of the understanding of the background of the disclosure and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.